In a conventional video encoder, most of the memory transfers and, as a consequence, a large part of the power consumption, come from motion estimation. Motion estimation consists in searching for the best match between a current block and a set of several candidate reference blocks according to a rate distortion criterion, a difference between the current block and a candidate reference block forming a residual error block.
The paper entitled “Rate Distortion Optimization for Video Compression”, by G. Sullivan, T. Wiegand, IEEE Signal Processing Magazine, pp. 74-90, November 1998 describes a method of computing a rate-distortion value. This value c is computed from an entropy h of the residual error block and on a reconstruction error mse derived from said residual error block, as given by equation (1):c=h+λ1*mse  (1)
where λ1 is a weighting coefficient.
This helps for selecting the best mode to encode the current block according to an expected bit-rate. The best reference block that is selected is the one that minimizes the rate-distortion value. Then the residual error block is entropy coded and transmitted with its associated motion vector and/or encoding mode.
But such a rate-distortion value is not optimal, especially in the case of a video encoder embedded in a portable apparatus having limited power.